I've been in touch with Qantas regarding this issue. Qantas issued the following statement:
As far as I can tell, the official Qantas policy at the moment is not an outright ban on arrivals access. However, lounge staff have a right to limit access during peak times, according to the
"Fly Well" policy and to ensure social distancing is kept. If the lounge staff determine there is additional capacity available in the lounge at the time, they can permit access on arrival - but may look ahead at the departing flight schedules to predict the number of people expected over coming hours. This may or may not explain why some people were denied entry when the lounge wasn't busy.
As I said in
my AFF article, it sure would be helpful if Qantas had a clear arrivals access policy that was published on its website! That would save a lot of confusion for both staff and customers...